1. Field of Invention
The disclosure relates to a laser device for the production of high-energy laser pulses.
2. Description of Related Art
A laser device is known from DE 41 02 409 C2 and includes the following elements, arranged one after another along the optical axis:                a outcoupling mirror,        a laser medium,        a phase-conjugate mirror (PCM) based on stimulated Brillouin scattering (SBS),        an end mirror.        
The outcoupling mirror and the end mirror form a start cavity, whereas the outcoupling mirror and the phase-conjugate mirror form a main cavity. With a phase-conjugate mirror, good beam quality is attained in mirror operation or operation of the main cavity. The individual light waves are precisely reflected back upon themselves and optical aberrations in the laser medium are not compounded, but almost compensated by reverse propagation of the light waves in the laser medium. The effect of a time-variable thermal lens occurs in the laser medium. That is, the laser medium functions as a time-variable thermal lens. However, this is balanced out by the phase-conjugate mirror, so that each laser pulse has good beam quality from the beginning.
In the laser device of DE 41 02 409 C2, a neutral density filter is positioned between the phase-conjugate mirror and the end mirror. The neutral density filter, which has a transmission of 20% in this example, reduces the Q-factor of the start cavity. The production of a single laser pulse of high energy occurs as described below. First the laser medium is pumped. To do so, a flash lamp is used for pumping. A laser beam builds up in the start cavity. The phase-conjugate mirror is still transparent, as the Brillouin threshold has not yet been reached. The pumping of the laser medium continues, increasing the strength of the laser beam within the start cavity. When, as a result of the pumping, high energy has been stored in the laser medium and the laser beam is further amplified, the Brillouin threshold is exceeded and the phase-conjugate mirror ignites. In a very brief period of time, in the nanosecond range, the phase-conjugate mirror changes from its initial very high transparency to mirror function. This starts the main cavity operation and the start cavity is decoupled. Since, following the ignition of the phase-conjugate mirror, the degree of reflection increases on a nanosecond time scale, the quality of the main cavity also increases in the same short period of time. This means that the phase-conjugate mirror also acts as a Q-switch in the main cavity. The high energy stored in the laser medium is converted in the main cavity into a laser pulse of high power and part of the laser pulse is emitted through the outcoupling mirror. The level of absorption of the neutral density filter is thus adjusted to the given pump energy, so that only a strong pulse is generated towards the end of the pumping process. For example, when pumping with a high power and long duration, a neutral density filter with a higher absorption is selected. This reduces the Q-factor of the start cavity, causing the phase-conjugate mirror only to ignite later, as required, at the end of the pumping process, in order to allow a single pulse to be generated.
A disadvantage of the laser device of DE 41 02 409 C2 is that, despite the generation of a pulse, which is stable in time, the delay period is difficult to predict. This is because the delay period, that is the time span between the beginning of the pumping process and the emission of the pulse, depends on many external parameters, such as, for example, the amount of the pump energy, the variation of pump power vs. time, or the absorption level of the neutral density filter that is used. The ambient conditions also play a role, such as the effect of mechanical vibrations and shocks. Therefore, the timing of the emission of the laser pulse cannot be precisely predicted, even if the beginning of pumping is known.
A further disadvantage is that the use of a neutral density filter of a particular absorption level also determines the energy of the pulse generated. For, as previously described, the pump power and the pumping progress are precisely adjusted to a certain absorption level of the neutral density filter, such that the laser pulse forms at the end of pumping.